Physical exertion is an important consideration when measuring exposure, because activity levels can affect how much of a pollutant a person inhales. In one study of different travel modalities, people riding bicycles inhaled more than 8 times as much air per minute as people driving cars and half again as much air as people who walked.8 Of course, the answer to avoiding exposures is not to exercise less—rather, smart technology may someday advise travelers on small behavioral tweaks (such as falling behind the traffic ahead or taking a slightly different route) that could significantly reduce exposure to pollutants.

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